Two spin-labeled acridines 9-(4-(2,2,6,6-tetramethyl-l-piperidinyloxyl) amino)-acridine and 6-chloro-9-(4-(2,2,6,6-tetramethyl-l-piperidinyloxy) amino)-2-methoxyacridine, have been prepared and their interaction with DNA and RNA studied by means of electron spin resonance. The nitroxide groups of both labels became highly immobilized on binding to DNA. In contrast, binding to RNA caused a much smaller effect on the motion of the nitroxide groups. These results suggest that the spin-labeled acridines will be useful probes for studying the conformation of nucleic acids in biologically important complexes such as chromatin.